设f(x)在区间[a,b]上满足a≤f(x)≤b,且有|f′(x)|≤q<1,令un=f(un+1). (n=1,2,…),u0∈[a,b],证明:级数(u-n+1-un)绝对收敛.

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问题 设f(x)在区间[a,b]上满足a≤f(x)≤b,且有|f′(x)|≤q<1,令un=f(un+1).
(n=1,2,…),u0∈[a,b],证明:级数(u-n+1-un)绝对收敛.

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答案因为|un+1-un|=|f(un)-f(un-1)|=|f′(ξ1)||un-un-1 ≤q|un-un-1|≤q2|un-1-un-2|≤…≤qn|u1-u0 [*]

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